Generation of high-current (1kA) subpicosecond electron single pulse

NUCLEAR INSTRUMENTS 8 MErHooS IN PHYSICS RESEARCH Nuclear

Instruments

and Methods

in Physics

Research

A 399 (1997) 180-184

SectIon A

Generation of high-current (1kA) subpicosecond electron single pulse T. Kozawa*, T. Kobayashi, T. Ueda, M. Uesaka Nuclear Engineering Research Laboratory.

Fact&

of Engineering, The Uniuersi@ of Tokyo. 2-22 Shirakata-Shirane, Iharaki 3 I9- I I. Japan

Received 2 October

Tokai-mura. Nab-gun.

1996; received in revised form 15 April 1997

Abstract An achromatic pulse compressor and a grid pulser system were designed for generation of a high-current single pulse in a subpicosecond time domain and were installed to the S-band (2856 MHz) linear accelerator of the University of Tokyo. 35 MeV electron pulses with the pulse width of 10 ps have been compressed by the achromatic magnetic pulse compression system. Generation of subpicosecond single-electron pulse with the charge of 1 nC has been succeeded. The shortest and average pulse widths in FWHM are 670 and 890 fs, respectively. The length of the pulse has been measured by a femtosecond streak camera with a time resolution of 2OOfs. No satellite of the main pulse could be observed. PACS: 29.17 &words: Subpicosecond;

Magnetic

pulse compression;

Linear accelerator;

1. Introduction Development of a short-pulse electron accelerator enables direct observation of ultrafast dynamics of electron-matter interaction. Pulse radiolysis is a very powerful method to detect and observe transient phenomena in radiation-induced reactions. The first experiment in the picosecond regime was carried out by the picosecond pulse radiolysis

* Correspondence address: The Institute of Scientific and Industrial Research, Osaka University, 8-l Mihogaoka, Ibaraki. Osaka 567, Japan. Tel.: +81 6 879 8.511; fax: +81 6 875 4346; e-mail: [email protected]. 0168-9002/97/$17.00 8 1997 Elsevier Science B.V. All rights reserved PI! SO1 68-9002(97)00920-O

Single pulse

system of Toronto University with a time resolution of 23 ps in the late 1960s [ 11. Since then, many researches using picosecond linacs have been reported on ultrafast phenomena in radiation chemistry, physics, biology and applied fields such as material science [24]. In 1977, a 10 ps electron single pulse was first generated at the 35 MeV S-band (2856 MHz) linear accelerator of Nuclear Engineering Research Laboratory (NERL), the University of Tokyo [S], and then the pulse radiolysis system was established [4,6,7]. Since then, ultrafast phenomena such as excitation, ionization and relaxation of atoms and molecules have been investigated in the picosecond regime with the time resolution of tens picoseconds.

T. Kozawa et al. JNucl. Instr. and Meth. in Phys. Res. A 399 (1997) 180-184

In the case of photo-induced reactions, the investigation on the ultrafast phenomena in the femtosecond region has started by using the femtosecond lasers [8,9]. On the other hand, the time resolution of the pulse radiolysis remains tens picoseconds since the late 1960s. Recently, generation of subpicosecond pulse (900 fs, 150 PC/pulse, 7.1 mm x 11 mm) was succeeded at the university of Tokyo [lo]. However, the charge was too little to detect radiation-induced reactions. We attempted to generate a high-current subpicosecond pulse by installing an achromatic pulse compressor and a new grid pulser system.

2. Experimental setup Fig. 1 shows the NERL 35 MeV electron linac. The NERL linac consists of a thermionic electron gun (Y-796, EIMAC), a 476 MHz ($ of the main accelerating microwave frequency of 2856 MHz) coaxial-typed subharmonic buncher (SHB), a 2856 MHz traveling wave type six cell prebuncher, accelerating tubes and focusing system. The accelerating potential of the electron gun is provided by 90 kV pulses of 8 us duration. The duration of its flat top is 4 ns. The velocity of the electron beam injected from the electron gun is modulated by the electric field at the gap of the SHB. In order to generate an isolated pulse without

any satellite, a fast rise time, low jitter trigger pulse synchronized with the accelerating RF waves is required. Up to now, a single pulse has been generated by the grid pulser, whose voltage is 300 V with the pulse width of 1 ns. The highvoltage new pulser (Kentech Co. Ltd.) was installed on a - 90 kV high potential deck. The output voltage of the new pulser is 1 kV and the pulse width is 1 ns. We attempted to increase the charge of emission from the Y-796 electron gun using this pulser. The achromatic pulse compressor was installed at the exit of NERL 35 MeV S-band linac as shown in Fig. 1. The achromatic pulse compressor consists of two 45” sector magnets and four quadrupole magnets. The upstream sector magnet was also used as an energy analyzer magnet. This linac has two accelerating tubes (ACCI and ACCII). The longitudinal distribution of electron pulse was modulated for pulse compression by adjusting the RF phase in ACCII. Pulse shape of emission from the electron gun with the energy of 90 keV was measured by a coaxial beam catcher at the distance of 25 mm from the anode. The time resolution of the coaxial beam catcher is less than 50 ps. The charge of emission was measured by a faraday cup. The signals were measured by the sampling oscilloscope (S-4, Tektronix Co. Ltd.) and DC microvolt ammeter (PM-18R, TOA Electronics Ltd.).

Grid Pulser

I

\

\

I

Prebuncher

Measuremeni

Fig. 1. S-band

electron

181

linear accelerator

of the University

Room

of Tokyo.

182

T. Kozawa et al.lNucl.

Instr. and Meth. in Phys. Rex A 399 (1997) 180-184

Streak Camera

Trajectory of Cherenkov Radiation Fig. 2. Magnetic

pulse compressor

and Cherenkov

Pulse width of a relativistic beam was evaluated by measuring Cherenkov radiation emitted by the relativistic electrons in air at the end of the beam lines. The Cherenkov radiation was measured by using the femtosecond streak camera which has the time resolution of 200 fs (FESCA-200, Hamamatsu Photonics Co. Ltd.). The optical measurement system is shown in Fig. 2. An optical band pass filter, which is centered at 465 nm and has a half-width of 12.5 nm, was used to avoid the pulse broadening due to optical dispersion in the convex lenses used in the measurement. All data were acquired by a single-shot measurement to avoid effects of jitter during accumulation. Beam sizes were also measured by using phosphor screens (AF995R, Desmarquest Co. Ltd.) at the end of beam lines and evaluated via image processing.

radiation

measurement

system.

energy spread were 19.1 MeV and 0.26%, respectively. The RF phase in ACCII was tuned so as to accelerate electrons in the early phase of the pulse more than those in the later phase of the pulse. The best RF phase in ACCII was adjusted so as to make the shortest pulse while monitoring its width by using the streak camera. The peak electric field in ACCII was adjusted to 10 MV/m. The charge passing through a @3 mm slit at the straight beam port was 700 pC/pulse. A typical measured pulse shape of compressed pulse riding on the phase of 72” is shown in Fig. 3. The pulse width was 850 fs and the

3. Results and discussion 3.1. Magnetic pulse compression system

at achromatic

In the magnetic pulse compression experiment, the RF phase in ACCI was tuned so as to minimize the energy spectrum of the pulse. Its energy and

Time (ps) Fig. 3. Measured pulse shape of compressed single pulse generated by using the previous grid pulser.

T. Kozawa et al.lNucl. Instr. and Meth. in Phys. Res. A 399 (1997) 180-184

Longitudinal

183

Beam sizes

phase space distribution and pulse width

I I

-10 Time (ps)

Time

-5 0 5 Time (ps)

longitudinal

x (mm)

(ps) At the end of the beam line

At the downstream exit of ACCB Fig. 4. Calculated

10

phase-space

distributions

and beam sizes after energy modulation

horizontal and vertical beam sizes of the compressed pulse were 3.3 and 5.8 mm, respectively. The charge was 208 pC/pulse. Also, 900 fs single pulse with the beam size of 3.3 mm x 3.0 mm was measured at the RF phase of 61.5”. Fig. 4 shows the results of simulation. The simulation parameters are 1007~mmmrad as 90% normalized emittance, 19.1 MeV as the energy and 0.26% as the energy spread at the exit of ACCI. Both the calculated pulse width and beam sizes are agreed with the experimental results.

2

and after pulse compression.

800

C 2 4

600

S .$ 400 s Z

200

Time

3.2. Installation

(ps) 1

of new grid pulser

When the previous grid pulser was used, the peak current of emission from the Y-796 electron gun was 1 A. The charge of the single pulse passing through the 52/3mm slit at the straight beam port was 700 PC/pulse. By increasing the applied voltage to the grid up to 900 V, the emission from Y-796 electron gun was increased up to 11 A with the pulse width of 1 ns. The calculated emittance of the emission using the computer code of E-Gun is about 1007~mm mrad as 90% normalized emittance. With the increase of the emission, the power in SHB was increased from 2.0 up to 2.8 kW so as to form a perfect single pulse. The electric field in the SHB cavity was increased from 2.4 up to

200

400

Time

600

800

1000

(ps)

Fig. 5. Measured pulse shape of original single pulse before compression generated by using the Kentech grid pulser.

T. Kozawa et al./Nucl. Instr. and Meth. in Phys. Res. A 399 (1997) 180-184

184

Fig. 6 shows a typical pulse shape of subpicosecond single pulse with the energy of 34.8 MeV passing through @3 mm slit. The charge was 1.04 nC. The charge of the compressed pulse passing through the 12(5mm slit was 1.36 nC. About 20% of the initial charge before compression was lost by the energy modulation. Furthermore, a part of the charge was lost at the slit at the end of the beam line. 0

10 Time (ps)

Fig. 6. Measured pulse shape of compressed the charge of 1 nC.

20

4. Summary single pulse with

2.9 MV/m. When a pulse width is short enough compared with one period of S-band microwave (350 ps) after SHB, a single pulse is accelerated finally. When an electron pulse is not sufficiently bunched by SHB, some of the electrons are accelerated at accelerating phases before and after the accelerating phase on which the main part of electrons rides. As a result, some satellites are observed at every 350 ps, especially at 350 ps before and after a main pulse. The satellites disturb the measurement of ultrafast phenomena induced by the main pulse, because satellites induce reactions, also. Fig. 5 shows the pulse shape at the end of beam line in the straight direction. Any satellite around the main pulse could not be observed within 10 ns before and after the main pulse. The charge of the original pulse passing through the a3 mm slit was 2 nC/pulse. The charge was increased by more than twice of the previous value. This pulse was compressed by achromatic pulse compression system. The width of the compressed pulse was 890 f 140 fs. The shortest pulse width was 670 fs.

35 MeV electron pulses with the pulse width of 10 ps was compressed by the achromatic magnetic pulse compression system at NERL of the University of Tokyo. 890 fs single pulse was generated with the charge of 1 nC through the 12/3mm slit. Any satellite around the main pulse could not be observed. By this improvement, the peak current rose from originally 200 A up to 1.1 kA at the end of the achromatic pulse compressor.

Cl1 M.J. Bronskill, W.B. Taylor, R.K. Wolff, J.W. Hunt, Rev. Sci. Instrum. 41 (1970) 333. PI C.D. Jonah, Rev. Sci. Instrum. 46 (1975) 62. c31 G. Beck, J.K. Thomas, J. Phys. Chem. 76 (1972) 3856. Y. Tabata, Chem. Phys. Lett. 64 c41 S. Tagawa, Y. Katsumura, (1979) 258. c51 Y. Tabata et al., J. Fat. Eng. Univ. Tokyo, B 34 (1978) 619. T. Ueda, T. Kobayashi, M. Washio, Y. C61 H. Kobayashi, Tabata, S. Tagawa, Radiat. Phys. Chem. 21 (1983) 13. S. Tagawa, J. Photoc71 Y. Yoshida. T. Ueda, T. Kobayashi, polym. Sci. Technol. 4 (1991) 171. PI C.K. Sun et al., Phys. Rev. B 48 (1993) 12365. c91 S.B. Fleischer et al., Appl. Phys. Lett. 62 (1993) 3241. [lOI M. Uesaka, T. Kozawa et al., Phys. Rev. E 50 (1994) 3068.